1. Field of the Invention
The invention relates to dielectric film useful as a substrate for flexible printed circuits and more particularly to dielectric films including indented areas of controlled depth for devices requiring films having at least one area less than 20 μm thick for optimum device performance.
2. Description of the Related Art
An etched copper or printed polymer thick film circuit pattern over a polymer film base may be referred to as a flexible circuit or flexible printed wiring. Originally designed to replace bulky wiring harnesses, flexible circuitry is often the only solution for the miniaturization and movement needed for current, cutting-edge electronic assemblies. Thin, lightweight and ideal for complicated devices, flexible circuit design solutions range from single-sided conductive paths to complex, multilayer three-dimensional packages. The use of flexible circuits is known, for example, in electronic devices including ink jet print heads, hard disk drive suspension assemblies and touch or finger sensors.
Commonly used dielectric film base materials for flexible electronic packaging include polyimide, polyester terephthalate, random-fiber aramid and polyvinyl chloride. Changes in electronic device design create the need for materials with properties or design possibilities surpassing previously attainable performance and processing capabilities. For example, a lower dielectric constant allows faster electrical signal transfer, good thermal performance facilitates cooling for a package, a higher glass transition or melting temperature improves package performance at higher temperature, and lower moisture absorption leads to signal and data processing at higher and higher frequencies. Additional improvement in device performance could be achieved by selective reduction of dielectric film thickness, but a cost-effective method is required to achieve the desired substrate thinning.
Polyimide film is a commonly used substrate for flexible circuits that fulfill the requirements of complex, cutting-edge electronic assemblies. The film has excellent properties such as thermal stability and low dielectric constant. Liquid crystal polymer (LCP) films represent suitable materials as substrates for flexible circuits having improved high frequency performance, lower dielectric loss, and less moisture absorption than polyimide films.
Current flexible circuits typically use dielectric substrate materials more than 25 μm thick. Automated handling and processing of films less than 50 μm thick is known to be difficult and, therefore, not cost effective. Flexible circuits such as flex-on-suspension circuits for hard disk drive devices would provide improved device performance if the flexible dielectric substrate could be thinner. Substrates may have uniform thickness or, for some applications, may benefit from thinning of only selected regions of the substrate. Selective substrate thinning, applied to ink jet printing heads could lead to improved image resolution with reduction in thickness of the material in the regions used for print head nozzle formation.
Dielectric films less than 25 μm thick may be obtained commercially, but processing of such thin films, whether of uniform or variable thickness, via multi-step conventional flexible circuit processing, causes mechanical damage or film stretching at any step in the process. Known flexible circuit processing techniques may not be capable of providing specified flexible circuits using dielectric substrate materials less than 25 μm thick. For further improvement in device performance there is a need for flexible circuits, including regions thinner than 25 μm, which can be mass produced to economically acceptable standards of yield and quality.